The present invention relates to an optical imaging device. The invention may be put to use in the context of microlithography which is used in the manufacture of microelectronic circuits. Accordingly, the invention also relates to a mask for use in this type of optical imaging device. Lastly, the invention relates to a method for determining an imaging error as well as to an imaging process that makes use of said method.
Particularly in the realm of microlithography, it is necessary among other things, and besides using components made with the highest precision possible, that the components of the imaging device, i.e. for example the optical elements such as lenses or mirrors, be positioned as precisely as possible in order to achieve a commensurately high image quality. The exacting quality requirements, which lie in the microscopic range in the order of magnitude of a few nanometers or less, are to a large extent a consequence of the continuing need to increase the resolution of the optical systems used in the manufacture of microelectronic circuits, in order to advance the miniaturization of the microelectronic circuits that are to be produced.
With the increased resolution which is normally accompanied by a shift to shorter wavelengths of the light being used, not only is a higher accuracy required in the positioning of the optical elements being used, but of course the requirements in regard to minimizing the imaging errors of the entire optical system are also increased.
In view of the short operating wavelengths in the UV range which are used in microlithography, for example operating wavelengths around 193 nm, but also in particular in the so-called extreme UV range (EUV) with operating wavelengths around 13 nm, it is often proposed, in order to satisfy the stringent requirements imposed on the positioning of the individual components, that the positions of individual components such as the mask table, the optical elements and the substrate table (for example a wafer table) be determined individually relative to a reference, for example a reference structure, which is often established in a so-called metrology frame, and that these components then be actively positioned relative to each other.
This solution has on the one hand the disadvantage that normally no real-time measurement of the position of the projection pattern of the mask on the substrate (in most cases a wafer) takes place, but that the relative positions of the components and the position of the image are only indirectly arrived at from the individual position data of the components relative to the reference. The respective measurement errors will add up cumulatively, possibly leading to a relatively high total measurement error. Furthermore, this involves a large number of elements that have to be actively positioned, all of which have to be set and controlled in their positions with the required angular accuracy in the nanorad (nrad) range and a translatory position accuracy in the picometer (pm) range. This further entails particularly severe requirements on the thermal stability of the reference and the supporting structure for the optical elements. Normally, only a few dozen nanometers per degree Kelvin (nm/K) can be tolerated here for the thermal expansion.
On the other hand, there are further a number of solutions known, where the position of the image of the projection pattern of the mask on the substrate is determined in real time. The position of the image of the projection pattern on the substrate can be corrected here with significantly fewer active elements, possibly even with only one active element. This simplifies not only the dynamic control of the rest of the components, the requirements to be imposed on the thermal stability of the reference and the supporting structure for the optical elements are also markedly less severe.
The real-time determination of the position of the image of the projection pattern of the mask on the substrate is often performed according to the so-called laser pointer principle. This involves directing a collimated laser beam along a path starting from a light source arranged in the area of the mask and proceeding near the path of the image-producing light through the optical elements participating in the formation of the image all the way to the area of the substrate, and to capture the laser beam with a detector in that place. Even the smallest deviation of the optical elements from their correct position will now cause a deflection of the laser beam from its target position which is registered through the detector and used to make a correction. A method of this kind has been disclosed for example in US 2003/0234993 A1 (Hazelton et al.).
With this arrangement it is possible, due to the fact that the laser beam is directed through the optical elements participating in the formation of the image, to determine not only possible deviations from the correct position of the image of the projection pattern of the mask on the substrate, but to also register other errors in the image (for example distortions, etc.). All of these position errors and other errors are herein collectively referred to as imaging errors.
The foregoing solution again has the disadvantage that the positioning of the light source and the detector are subject to correspondingly stringent requirements in regard to angular accuracy in the nanorad range (nrad) and translatory accuracy in the picometer range (pm) as well as in regard to thermal stability. Most of all, the light source normally has to be supported by an elaborate thermally stable supporting structure which is normally arranged far from the reference (for example in a so-called metrology frame). In addition, expensive measures are again necessary to determine the positions of these components relative to a reference, with the result also entering into the calculation of the correction.